The present invention relates to a calibration system, calibrating method, and correction processing program for color correction.
Image forming apparatus equipped with a color printing function, such as a copying machine and a composite machine (MFP: Multi Function Peripheral), has been used widely. In this image forming apparatus, the following process is conducted: image information is generated based on print data transmitted from a host computer; an electrostatic latent image is formed on a photoreceptor that has been electrically charged by irradiating light based on the image information; the electrostatic latent image is developed into a toner image by the adhesion of electrically charged toner; and the toner image is transferred onto a sheet through an intermediate transfer member, such as a transfer roller and a transfer belt.
In the case of printing a color image, in order to reproduce the color displayed on a monitor of a host computer accurately, the host computer conducts color correction (referred to calibration) by the use of a color correction table. This color correction table can be produced with the following processes. Firstly, patterns (hereafter, referred to measurement patches) in which densities are changed for each color of toners of Yellow, Magenta, Cyan, and Black are printed. Secondly, the printed measurement patches are subjected to color measurement by a calorimetric device. Thirdly, the obtained calorimetric values are processed to produce the color correction table.
However, in an image forming apparatus to print the measurement patches, periodic density unevenness or accidental density unevenness may occur due to rotational unevenness of a photoreceptor or an intermediate transfer member. Therefore, at the time of printing a sheet (hereafter, referred to a patch sheet) on which the measurement patches are arranged in a predetermined direction, if such density unevenness occurs, the problem that the precision of a calibration gets worse, eventually arises.
Then, conventionally, a method has been used to suppress the influence of density unevenness by printing plural patch sheets, conducting color measurement for the printed plural patches, and calculating the average values of the calorimetric values.
Moreover, periodic density unevenness due to rotational unevenness of a photoreceptor or an intermediate transfer member is apt to take place in a direction vertical to a sheet feeding direction (that is, a belt-shaped density unevenness is apt to take place along a direction vertical to a sheet feeding direction). Then, a method has been used to suppress the influence of density unevenness by arranging measurement patches of each color in a sheet feeding direction (refer to Japanese Patent Unexamined Publication No. 2001-94803).
However, in the method of calculating the average values of the calorimetric values, since it is necessary to print plural patch sheets, there is a problem to consume sheets uneconomically. Moreover, when density unevenness occurred in the same position, even if the average values of the calorimetric values are calculated, the influence of the density unevenness cannot be suppressed. As a result, there is a problem that the precision of calibration gets worse.
Moreover, the method of Japanese Patent Unexamined Publication No. 2001-94803 merely avoid the bias of the periodic density unevenness to one color and does not perform correction for the occurred density unevenness. Therefore, there is a problem that the influence of the density unevenness appears on all the colors. Further, accidental density unevenness does not always occur in a direction vertical to a sheet feeding direction. Therefore, in the case that density unevenness occurs in a sheet feeding direction, all the measurement patches of a certain color may be printed thinly. As a result, there is a problem that the precision of calibration gets worse.